The latest bomber flights around the island were the fourth time in the past three years that Russian bombers circumnavigated Guam.

Earlier incursions took place on Dec. 13 and Nov. 12, 2014, and Feb. 12, 2013. During the 2013 incident, US F-15 jets were scrambled to intercept the bombers.

Eastburn declined to specify the exact type of bombers involved in the circumnavigation and sought to play down the incident, noting that the flights "in no way" violated U.S. airspace around the island.

Other officials said the bombers were Tu-95 Bear H nuclear-capable bombers.

One defense official said Japanese jet fighters intercepted the bombers during an earlier stage of the Guam overflight.

Brian McKeon, principal deputy undersecretary of defense for policy, told a House hearing Wednesday that Moscow is building up its military forces and engaging in nuclear saber rattling, including threats to use nuclear weapons.

He added that "reckless" statements about using nuclear arms "cause us to wonder whether Russia continues to respect the profound caution that world leaders in the nuclear age have shown with regard to the brandishing of nuclear weapons or nuclear-inspired rhetoric."

In response to Russian provocations, the Pentagon is developing new unmanned systems, a new long-range bomber, a new long-range stand-off cruise missile, and a number of innovative technologies, McKeon said in testimony to a joint hearing of the House Armed Services and Foreign Affairs Committees.

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The pre-Thanksgiving Day flights by the Bear bombers come amid an increasing number of provocative Russian bomber activities in Asia, near US coasts, and over Europe.

On Oct. 27, two Tu-142 Bear F maritime reconnaissance aircraft, a variant of the Bear H, flew within one mile of the aircraft carrier USS Ronald Reagan in the Sea of Japan.

During the Reagan incident, four Navy F/A-18 jets were dispatched to escort the Bear bombers that were flying at an altitude of 500 feet.

The bomber incursions are part of stepped-up Russian attempts at nuclear intimidation of the United States that have included increased bomber flights near US coasts and in at least one case a simulated nuclear cruise missile attack, according to defense officials.

On July 4, two Bear bombers came within 40 miles of the California coast in what US officials said was an incident timed to the Fourth of July holiday. One of the Russians radioed "happy birthday" to an intercepting U.S. jet during that encounter.

Tu-95s are known to carry Russia's new long-range Kh-55SM cruise missiles, which can be armed with either nuclear or conventional warheads and have a range of up to 1,800 miles.

The command said in a statement that no US jets were dispatched to intercept the Russian bombers, since the aircraft were flying in international airspace.

The aircraft were identified by other defense officials as Bear H strategic bombers.

Russia conducted its operations around Guam in the past with Tu-95 long-range bombers.

"We do not typically discuss the type of aircraft in these situations, but what we can confirm is that the aircraft were flying safely in international airspace and in accordance with international norms," the command statement said. "As such, the decision was made not to intercept them."

The 36-mile-long island is home to Andersen Air Force Base, which regularly hosts deployments of B-52 bombers and temporary deployments of B-2 stealth bombers and F-22 advanced jet fighters. The Global Hawk long-range drone is also based on Guam.

The island also hosts a major Navy base where four attack submarines are deployed.

Guam is located some 3,800 miles west of Hawaii and will become the new home for 5,000 US troops being relocated from Japan over the next seven years.

It is considered America's most significant military outpost in Asia and a key element of the Obama administration's shift of forces toward Asia.

The aircraft at Andersen Air Force Base, Guam. Jason Robertson/US Air Force

Russia has announced that it will also be seeking to conduct a pivot to Asia by building up its forces in the Pacific. Russian military forces in the Pacific have declined sharply since the collapse of the Soviet Union in 1991 and are being built up under President Vladimir Putin. Analysts say the Russians are also seeking to gain control over the resource-rich Arctic region from bases in the Pacific.

Russia announced this week that it is building 400 new military facilities on two Russian-controlled islands claimed by Japan, as part of a strategy to build up forces in the region, Japan's NHK reported.

Rep. J. Randy Forbes (R-Va.), chairman of the House Armed Services subcommittee on sea power, said that Putin "seems intent on recreating Cold War-era tension between Washington and Moscow, right down to the same antiquated Bear bomber patrols."

"Whether it is fomenting unrest in Eastern Europe, complicating efforts to resolve Syria's brutal civil war, or supporting rogue regimes around the world, Russia has recently opted to play a destabilizing role in world affairs," Forbes said. "Futile provocations only serve to deepen Russia's growing isolation from the rest of the international community with no appreciable gain."

Mark Schneider, a former Pentagon strategic arms policymaker at the National Center for Public Policy, said that the latest flights suggest that Moscow views the United States and NATO as its main enemies.

"This has been made even clearer in their recent doctrinal publications concerning their military and naval doctrine," said Schneider.

Sergei Karpukhin/Reuters

"They also believe that Russian clout is enhanced by direct and indirect nuclear threats," he said. "Flying nuclear capable bombers into air defense identification zones is one of their standard nuclear threats."

According to Schneider, Russia announced years ago that Guam is a key target of bomber flights, and several years ago in the context of a bomber flight around Guam, Russia announced that heavy bombers would now take on the mission of attacking aircraft carriers.

A B-52 bomber from Guam recently flew over the disputed South China Sea, where China is aggressively claiming 90 percent of the strategic waterway as its maritime territory.

Eastburn, the Pacom spokesman, said of the bomber flight that "international airspace belongs to everyone and is not the dominion of any single nation."

"The United States matches our words and our diplomacy with operations like this one conducted by Russia," he said. "The rights, freedoms, and uses of the sea, air, space, and cyberspace guaranteed to all nations in international law are essential to prosperity, stability, and security in the Indo-Asia-Pacific, and we encourage all nations to exercise within the parameters of those laws."

Navy and Air Force facilities on Guam will be getting a multimillion-dollar upgrade in the coming months.

Steven Wolborsky, director of plans, program, and readiness at Andersen told Stars and Stripes last month that Guam's two 11,000-foot runways were rebuilt in the past decade and that the base can handle more than 155 aircraft. Some 19 million pounds of explosives are stored on the base.

Six B-52s are deployed continuously for six-month periods, while jet fighters are rotated every four months. Five Global Hawks are based at Andersen.